The reflow process is usually employed for soldering an electronic part to a printed circuit board, or the like. The reflow process is a process for soldering an electronic part to a printed circuit board by applying a solder cream to an area of the printed circuit board to which the electronic part is to be soldered, mounting the electronic part so that its electrode and planar lead may engage the applied area, and heating the printed circuit board in a ref low oven to melt the solder cream.
The solder cream used in the ref low process is a viscous solder material obtained by kneading a solder alloy powder and a flux paste. The solder alloy powder which has usually been employed for a solder cream has been of an Sn—Pb eutectic alloy.
This solder alloy containing lead has been restricted in use as lead pollutes the environment and enterprises have been actively trying to put the so-called “lead-free solder” into practical use. The solder used for a solder cream is also being replaced by a “lead-free solder”. A typical lead-free solder material is an SnAg solder material (eutectic containing 3.5% Ag) and it has a melting point of about 220° C.
The Japan Electronics and Information Technology Industries Association (JEITA) published a road map for putting a lead-free solder into practical use in November, 2001 based on data accumulated by enterprises entrusted by NEDO from their research and development project for the practical use of a lead-free solder, and recommended therein Sn-3.0Ag-0.5Cu as the preferred composition of an SnAgCu material. Such being the case, many of the SnAgCu lead-free solders currently available in Japan are of the Sn-3.0Ag-0.5Cu composition.
A wide variety of patent applications relating to lead-free solders have been filed by various companies (see, for example, Patent Documents 1 to 3).
Patent Document 1 relates to an SnAgCu lead-free solder containing Sn-3.0Ag-0.5Cu and discloses the following. According to the Abstract in Patent Document 1, it aims at “providing a lead-free solder paste not forming any void in a soldered portion for a package part even by soldering at the reflow temperature not higher than 250° C. at which no thermal damage occurs to an electronic part or printed circuit board during the soldering of a surface-mounted part, not causing any chip standing of a chip part, and excellent in printing property.” As a solution, it discloses that “two or more kinds of solder alloy powders different in composition or the proportions of its components are mixed and melted so that the two or more kinds of solder alloy powders composed of 0-8 mass % Ag, 0-5 mass % Cu and 80-100 mass % Sn may produce a solder paste containing 1-5 mass % Ag and 0.5-3 mass % Cu with a balance consisting of Sn.”
Patent Document 2 aims at “improving an Sn—Ag alloy and providing an Sn—Ag solder alloy having excellent strength, thermal stability and good bonding property”. As a solution, it discloses that “it contains tin as its main constituent and further contains 1.0 to 4.0% by weight of silver, not more than 2.0% by weight of copper, not more than 1.0% by weight of nickel and not more than 0.2% by weight of phosphorus. It may alternatively contain tin as its main constituent and further contain 1.0 to 4.0% by weight of silver, not more than 2.0% by weight of copper, not more than 1.0% by weight of nickel and not more than 0.1% by weight of germanium. Cu forms a solid solution in Sn and improves the strength and heat resistance of the alloy without impairing its wetting property. Ni raises the thermal stability of the alloy because of its high melting point. Adding Ni also forms a fine crystal structure or forms an Ni—Sn compound and thereby yields an alloy of improved strength and thermal fatigue property. Adding P and Ge forms a thin oxide film upon melting of the solder and thereby suppress the oxidation of solder components such as Sn.”
Patent Document 3 aims at “modifying an Sn—Ag—Cu eutectic alloy with a small amount of one or more low-cost alloy additives to enhance the high temperature stability of its microstructure and its thermal-mechanical fatigue strength without decreasing solderability”. As a solution, it discloses that “an appropriate fourth or fifth elemental additive in a total amount not exceeding about 1% by weight is added to an Sn—Ag—Cu ternary eutectic solder alloy consisting mainly of Sn, 4.7% Ag and 1.7% Cu (wt. %), while a modifier of the intermetallic interface between the solder and substrate is selected from the group consisting of Ni, Fe and other elements acting like Ni or Fe, to improve the high temperature stability and the thermal-mechanical fatigue strength of the microstructure of a solder joint.”
The soldering of an electronic part to a printed circuit board, or the like by the reflow process is often carried out by a method employing at least two kinds of solders having different reflow temperatures and conducting two steps of soldering on the high- and low-temperature sides, respectively, i.e. the so-called dual-temperature solder connection. For example, a semi-conductor chip is joined to the board by a high-temperature step, while wiring connections to the printed circuit board are made by a low-temperature step, so that it may be possible to prevent the joint of the semiconductor chip to the board from melting and failing to maintain its joining, or from even its separation.
In the case of lead-containing solders, a lead-rich high-temperature solder (having a melting point of 275° C. to 302° C.) is, for example, used for the high-temperature step, and a lead-tin eutectic solder (having a melting point of 183° C.) for the low-temperature step. In the event that a lead-free solder, for example, an Sn—Ag solder alloy, is used for the low-temperature step of dual-temperature solder connection, a suitable substitute for the lead-containing high-temperature solder is required as a solder material for the high-temperature step.
Although an Au—Sn alloy (having a eutectic melting point of 280° C.) is available as a high-temperature lead-free solder material which can be used for dual-temperature solder connection, it is expensive and there is no practical solder material that can be used widely. A silver brazing material is available as a metallic joining material, but requires a joining temperature which is as high as 800° C. to 900° C. It is known that silver nano-particles have activated surfaces and require only a joining temperature of 250° C. to 300° C. However, silver nano-particles are usually coated with an organic material and mixed with an organic solvent prior to their use to avoid the oxidation of their surfaces. They form a joint having problems of voids and an organic residue. The voids and organic residue lower the thermal conductivity, electrical conductivity and thermal fatigue reliability of the joint which are important for an electronic appliance. Therefore, they undesirably require a reducing atmosphere for the organic material, the application of a load, etc. to be capable of functioning as a joining material.
Patent Document 4 discloses an invention of an electronic appliance for which a special solder paste is used as a high-temperature lead-free solder.
Patent Document 4 aims at “providing an electronic device made by an entirely novel solder connection, and more specifically, realizing a flip-chip connection for the high-temperature step of dual-temperature solder connection as a substitute for a high-Pb solder containing a large amount of Pb”. As a solution, it discloses “an electronic device characterized in that joints between the electrodes of electronic parts and the electrodes of a substrate are formed by connecting metal balls containing a simple metal, alloy, compound or a mixture thereof with either Sn or In”, and “an electronic device characterized in that joints between the electrodes of electronic parts and the electrodes of a substrate are formed by supplying the clearances between the electrodes with a paste obtained by mixing metal balls containing a simple metal, alloy, compound or a mixture thereof with one or more of an Sn—Cu solder, an Sn—Ag solder, an Sn—Ag—Cu solder and a solder prepared by adding one or more of In, Zn and Bi thereto, and heating them to melt components of the solder balls to join the metal balls to each other and also between the electrodes of the electronic parts and the electrodes of the substrate”.    Patent Document 1: Publication JP-A-2002-126893    Patent Document 2: Publication JP-A-11-77366    Patent Document 3: Publication JP-T-2001-504760    Patent Document 4: Publication JP-A-2002-314241